Ithilien was a region and fiefdom of Gondor.
It was a fair and prosperous land during the Second Age and the first part of the Third Age, when Gondor was strong and Mordor deserted. Of old its chief city was Minas Ithil, the Tower of the Moon, but when this was captured by Mordor it was renamed Minas Morgul, the Tower of Black Sorcery. After this event, the majority of the people of Ithilien fled across the Anduin to escape war, but the Stewards of Gondor still kept scouts there, operating out of secret locations such as Henneth Annûn. Those who stayed fled in T.A. 2954 when Mount Doom erupted.
During the War of the Ring, in early May T.A. 3019, Gollum leads Frodo Baggins and Samwise Gamgee through Ithilien on their way to Cirith Ungol and into Mordor. After witnessing an ambush of Haradrim, the hobbits were found by Faramir, the son of the Steward Denethor, but are allowed to continue when he is satisfied they are not agents of Sauron.
During the Fourth Age, Ithilien was ruled by the Princes of Ithilien, a line that started with Faramir and Éowyn, who became known as the White Lady of Ithilien. Minas Morgul was not repopulated, and Faramir had his throne in Emyn Arnen. After the fall of Sauron, Legolas brought elves from Greenwood, and they dwelt in Ithilien, and once again it became the fairest country in all the westlands.